So at the moment your guinea pig is larger than your baby rabbit? Is your baby rabbit still with it's mother?
I am assuming not.
Do you mind me asking: Are you actually asking if you can keep a baby rabbit with a guinea pig as a temporary thing? Or do you wish this to be permanent?
Do you have one lonely guinea pig and are you trying to give it companionship? Or do you have a baby rabbit without a mother and you are trying to give it comfort with the guinea pig?
I ask so that we can help to give you the best advice for your guinea pig AND your rabbit. One week old seems very young for a rabbit to be without its mother.
Apart from the bacteria that many rabbits carry (harmless to rabbits, fatal to guinea pigs), as they grow rabbits become much stronger than guinea pigs. Even those rabbits who really love their guinea pig companions can injure them accidentally. As
@Swissgreys says, some rabbits turn on the guinea pigs, but even when they never, ever attack, both rabbits and guinea pigs are prone to suddenly running to a dark corner when something surprises them - a noise, a smell, a shadow, these can all cause sudden panic in both animals. As rabbits kick off with their back legs as a reaction to fright, if the guinea pig happens to be sat behind the rabbit it can be seriously injured or even killed by this.